Billings, S. A. | bioRxiv Science

bioRxiv2025

Continental-scale relationships of fine root and soil carbon stocks hold in grasslands but not forests

Increasing root carbon inputs into soils has been proposed as a solution to increasing soil organic carbon (SOC). However, while fine root carbon (FRC) inputs can increase SOC accrual in soils, FRC can also enhance SOC loss by stimulating microbial respiration and cause a net loss of SOC through priming. It remains unclear how SOC varies as a function of FRC at broad spatial scales and across ecosystems and depths. Here, we tested the relationship of SOC and FRC using data from 43 sites across the US National Ecological Observatory Network (NEON). We found that total stocks of SOC and FRC in the top 2 meters of soil were positively related with an across-ecosystem slope of 7 {+/-} 3 kg SOC m-2 per kg FRC m-2. However, grassland sites primarily drove this relationship. Grasslands had 15 {+/-} 2 kg SOC m-2 per kg FRC m-2, which is double the across-ecosystem slope. We used deviations from the standardized 1:1 relationship between FRC and SOC to infer whether ecosystems were net priming (indicated by observed SOC being lower than the 1:1 line) or SOC accruing (higher SOC than the 1:1 line). Grassland soils and especially their deep soil layers (>30 cm) showed primarily SOC accrual with increasing fine root abundance. Meanwhile, forest soils had high variability in whether increasing fine roots were associated with net SOC priming or accrual across both shallow and deeper soil layers. We found that in grasslands, FRC inputs are strongly related to SOC accrual, especially at depth and at sites with high moisture and clay content. In contrast, SOC-FRC relationships in forests remain difficult to characterize. Nevertheless, deep grassland soils may serve as optimal environments in which increasing FRC could lead to meaningful increases in SOC stocks.

ecology↗

bioRxiv2019

Ecological and genomic attributes of novel bacterial taxa that thrive in subsurface soil horizons

While most bacterial and archaeal taxa living in surface soils remain undescribed, this problem is exacerbated in deeper soils owing to the unique oligotrophic conditions found in the subsurface. Additionally, previous studies of soil microbiomes have focused almost exclusively on surface soils, even though the microbes living in deeper soils also play critical roles in a wide range of biogeochemical processes. We examined soils collected from 20 distinct profiles across the U.S. to characterize the bacterial and archaeal communities that live in subsurface soils and to determine whether there are consistent changes in soil microbial communities with depth across a wide range of soil and environmental conditions. We found that bacterial and archaeal diversity generally decreased with depth, as did the degree of similarity of microbial communities to those found in surface horizons. We observed five phyla that consistently increased in relative abundance with depth across our soil profiles: Chloroflexi, Nitrospirae, Euryarchaeota, and candidate phyla GAL15 and Dormibacteraeota (formerly AD3). Leveraging the unusually high abundance of Dormibacteraeota at depth, we assembled genomes representative of this candidate phylum and identified traits that are likely to be beneficial in low nutrient environments, including the synthesis and storage of carbohydrates, the potential to use carbon monoxide (CO) as a supplemental energy source, and the ability to form spores. Together these attributes likely allow members of the candidate phylum Dormibacteraeota to flourish in deeper soils and provide insight into the survival and growth strategies employed by the microbes that thrive in oligotrophic soil environments.\n\nImportanceSoil profiles are rarely homogeneous. Resource availability and microbial abundances typically decrease with soil depth, but microbes found in deeper horizons are still important components of terrestrial ecosystems. By studying 20 soil profiles across the U.S., we documented consistent changes in soil bacterial and archaeal communities with depth. Deeper soils harbored distinct communities compared to the more commonly studied surface horizons. Most notably, we found that the candidate phylum Dormibacteraeota (formerly AD3) was often dominant in subsurface soils, and we used genomes from uncultivated members of this group to identify why these taxa are able to thrive in such resource-limited environments. Simply digging deeper into soil can reveal a surprising amount of novel microbes with unique adaptations to oligotrophic subsurface conditions.

microbiology↗